There are various types of (inner rotor type and outer rotor type) multiphase step motors, e.g., PM type motor having a permanent magnet rotor, VR type motor having a gear-shaped iron rotor, hybrid (HB) type motor having a rotor constituted by a gear-shaped iron core and a permanent magnet. The PM type multiphase step motor has a plurality of stator units layered in an axial direction and a permanent magnet rotor, in which N-poles and S-poles are alternately formed and faced the stator units. By switching directions of current flowing through coils of the stator units, stator magnetic poles and rotor magnetic poles mutually attract and repel, so that the rotor can be rotated.
In the multiphase step motor, number of the layered stator units is equal to that of phases. With this structure, when the direction of the current flowing through the phase coil is changed, current values of the stator units are transitionally unbalanced, so that strength of action forces and reaction forces generated between the stator and the rotor are varied and vibration is generated. To reduce torque ripple generated in a rotational direction, a PM type multiphase step motor, in which shaped of adjacent stator magnetic poles are mutually made different (see Patent Document 1), and a PM type multiphase step motor, in which a relationship between positions of stator magnetic poles and a pitch of magnetic poles of a permanent magnet is made different in each of phases to be excited (see Patent Document 2), etc. have been developed. In the above described multiphase step motors, the variation of magnetic flux between the stator magnetic poles and the permanent magnet is smooth.
Patent Document 1: Japanese Patent Gazette No. 2002-204562
Patent Document 2: Japanese Patent Gazette No. 11-55927